The present invention relates to a numerically controlled cutting method, and more particularly to a numerically controlled cutting method of cutting a curved surface generated by connecting corresponding points on two curved lines with a milling machine or a wire-cut electric-discharge cutting machine.
Wire-cut electric-discharge cutting machines have upper and lower guides and a wire extending therebetween and kept taut for producing an electric discharge between the wire and a workpiece to cut the latter. The workpiece is fixed to a table and is moved in the directions of X and Y in the pattern of a shape to be cut under the commands from a numerical control unit. With the wire extending perpendicularly to the table (workpiece), shapes cut on the workpiece are identical on its upper and lower surfaces. Where the upper guide is displacable in the direction of X and Y (referred to as "U-axis" and "V-axis"), and when the upper guide is displaced in a direction normal to the direction of travel of the workpiece to incline the wire with respect to the workpiece, shapes cut on the upper and lower surfaces of the workpiece become different, forming inclined surfaces cut by the wire, a process known as so-called taper cutting.
FIG. 1 is a diagram illustrative of such taper cutting. A wire WR extends and is kept taut between an upper guide UG and a lower guide DG, the wire being inclined at a given angle with respect to a workpiece WK. Let the lower surface PL of the workpiece WK be of a programmed shape (the upper surface QU of the workpiece WK may alternately be of the programmed shape), the taper angle be .alpha., the distance between the upper and lower guides UG, DG be H, and the distance from the lower guide DG to the lower surface of the workpiece WK be h. Then the amount of offset d1 of the lower guide DG with respect to the lower workpiece surface PL and the amount of offset d2 of the upper guide UG with respect to the lower workpiece surface PL can be expressed respectively by: ##EQU1## where d is the width of the cut.
Accordingly, by moving the upper guide UG which supports the wire WR to keep the amounts of offset d1, d2 constant as the workpiece moves, the taper cutting can be carried out with the taper angle .alpha., as shown in FIG. 2. The upper and lower guides UG, DG move along a dotted line and a dot-and-dash line, respectively, in FIG. 2. Commands for such wire-cut electric-discharge cutting include a command for a programmed path along the upper or lower workpiece surface, a command for the speed of feed along the programmed path, a command for the taper angle .alpha., commands for the distances H, h, and the like for cutting the workpiece according to the commands.
With the taper cutting heretofore practiced, the taper angle is fixed, and cutting with continuously variable taper angles cannot be effected. In particular, the workpiece cannot be cut to produce completely different shapes on the upper and lower surfaces of the workpiece. For example, it has been impossible to cut the workpiece along a straight line on the upper surface and along an arcuate line on the lower surface. If wire-cut electric-discharge cutting were capable of such cutting, it would be applicable to cavity cutting such as for blanking dies and plastic dies, and NC wire-cut electric-discharge cutting machines would find a much wider range of applications. If the above wire-cut electric-discharge cutting were rendered possible, it would similarly be possible to use the shank of a milling cutter BT on a milling machine in cutting a workpiece WK as shown in FIG. 3(a), an arrangement which could increase the cutting efficiency as compared with the conventional practice (FIG. 3(b)) which employs only the tip of the milling cutter for workpiece cutting.
With the foregoing in view, it is an object of the present invention to provide a numerically controlled cutting method capable of cutting a curved surface having different upper and lower shapes, that is, a curved surface generated by connecting corresponding points on two curved lines.